In situ combustion process



US. Cl. 166-256 11 Claims ABSTRACT OF THE DISCLOSURE This specification discloses an in situ combustion process in which the production Well or wells are alternately choked and opened to eflect an increase in oil recovery. The production well is choked, preferably to the extent of shutting it in, until an increase in bottomhole pressure of at least atmospheres is obtained. Thereafter, the production well is opened and hydrocarbon fluids are recovered therefrom as the bottomhole pressure declines. Preferably, the choking step is instituted after gas permeability is established between the injection and production wells. Also, the choking and opening steps may be repeated at appropriate intervals during the process.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to the recovery of oil from subterranean reservoirs, and more particularly to a new and improved in-situ combustion process.

DESCRIPTION OF THE PRIOR ART In the recovery of petroleum crude oils from subterranean reservoirs it usually is possible to recover only a minor portion of the oil originally in place in a reservoir by the so-called primary recovery methods, i.e., those methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. In these supplemental techniques which are commonly referred to as secondary recovery operations, although they may be primary or tertiary in sequence of employment, energy is supplied to the reservoir as a means of moving the oil in the reservoir to suitable production wells through which it may be withdrawn to the surface of the earth. Perhaps the most common secondary recovery processes are those in which displacing fluids such as Water or gas are injected into an oil-bearing reservoir in order to displace the oil therein to suitable production wells. Other widely known secondary recovery processes are the so-called huff and puff gas injection techniques such as the procedure disclosed by US. Patent No. 3,123,134 to I. R. Kyte et al. In this procedure the reservoir typically is closed off to production and a suitable gas, such as air, natural gas, combustion products, etc., is injected into the reservoir. Thereafter, gas injection is terminated and the reservoir is placed on production through the wells used for gas injection and/ or additional production wells.

Another secondary recovery process which is showing increasing promise is the concurrent or forward burn insitu combustion technique. In this procedure a portion of the reservoir oil is burned or oxidized in-situ to create a combustion front. This combustion front is advanced through the reservoir in the direction of one or more production wells by the injection of a combustion-supporting gas through one or more injection wells. The combustion front is preceded by a high temperature zone, commonly called a retort zone, within which the reservoir oil is heated to effect a viscosity reduction and is subjected to distillation and cracking. Hydrocarbon fluids including the 3,434,541 Patented Mar. 25, 1969 heated, relatively low viscosity oil and the distillation and cracking products of the oil then are displaced to production wells where they are withdrawn to the surface of the earth. The in situ combustion procedure is particularly useful in the recovery of thick, heavy oils such as viscous petroleum crude oils and the heavy, tar-like hydrocarbons present in tar sands. While these tar-like hydrocarbons may exist as solid or semi-solid materials in their native state, they undergo a sharp viscosity reduction upon heating and in an in situ combustion process behave like the more conventional petroleum crude oils.

In in situ combustion oil recovery procedures various techniques have been proposed which involve the manipulation of one or more production wells in the recovery pattern. These techniques typically are for the purpose of controlling the movement of the combustion front or the flow of fluids wtihin the formation, particularly those fluids in the vicinity of the retort Zone and combustion zone. Thus, in US. Patent No. 2,390,770 to Barton et al. there is disclosed a procedure for controlling the movement of the combustion front by such procedures as throttling, to the extent if necessary of closing, a production well toward which the combustion front is preferentially moving and/ or injecting various fluids such as drilling mud or water into such a well. Also, in U.S. Patent No. 2,862,557 to van Utenhove et al. there is disclosed an in situ combustion process in which gas is injected through a production well in order to bring about a pressure gradient reversal within the formation so as to force condensed products away from the production well into a heated portion of the formation.

SUMMARY OF THE INVENTION In accordance With the present invention, there is provided a new and improved concurrent in situ combustion process for the recovery of hydrocarbon fluids from a subterranean oil reservoir such as a tar sand or a more conventional petroleum crude oil formation. In carrying out the invention, a combustion front is established in the reservoir and advanced through the reservoir in the direction of a production well by introducing a combustion-supporting gas through an injection Well. Hydrocarbon fluids are recovered from the reservoir through the production well by any suitable technique. Thereafter, the production well is choked until the bottomhole pressure thereof is increased by at least 10 atmospheres over the bottomhole pressure existing at the time the well was first subjected to choking. The production well then is opened in order to allow the bottomhole pressure thereof to decline and hydrocarbon fluids are withdrawn therefrom. By this procedure the amount of oil recovered from the reservoir may be increased over that recovered by a conventional in situ combustion process.

In one embodiment of the invention the production well is choked, as described above, after gas permeability is established between the injection well and the produc tion Well. In a further embodiment of the invention the steps of opening and choking the well are repeated during the in-situ combustion drive.

DESCRIPTION OF SPECIFIC EMBODIMENTS The present invention is carried out utilizing one or more injection Wells and one or more production wells extending from the surface of the earth into the subterranean reservoir. The injection and production wells may be located and spaced from one another in any desired pattern. For example, the line drive pattern may be utilized in which a plurality of injection wells and a plurality of production wells are arranged in rows which are spaced from one another. Exemplary of other patterns which may be used are the so-called circular drive patterns in which a plurality of production wells are spaced about a central injection well. Typical circular drive patterns are the inverted five-spot, seven-spot, and nine-spot patterns. The above and other patterns for effecting secondary recovery operations are well known to those skilled in the art. For a more detailed description of such patterns, reference is made to Uren, L. C., Petroleum Production Engineering-Oil Field Exploitation, 3d ed, McGraw- Hill Book Company, New York, Toronto, London, 1953, and more particularly to the section entitled Arrangement of Injection and Production Wells in Waterfiooding, appearing at pages 528-532. While the well patterns described in Uren are with reference to waterflooding operations, it will be recognized that such patterns are also applicable to in situ combustion procedures.

For the purpose of simplicity in describing the invention, reference sometimes will be made herein to only one injection Well and one production well in a recovery pattern. However, it will be recognized that in practical applications of the invention a plurality of such wells, particularly the production wells, may be and in most cases will be utilized.

In practicing the invention, a concurrent in situ combustion drive is instituted within the reservoir by any suitable technique. Combustion may be initiated adjacent the injection well by locating an electrical or gas-fired heater within the Well and introducing a suitable combustion-supporting gas such as air through the well. Also, in some reservoirs the combustion front may be established by auto-oxidation. Thus, air, which may be enriched with oxygen, may be injected through the injection well in order to slowly bring the reservoir oil up to the combustion temperature without the use of extraneous heating means.

Normally, combustion will be initiated immediately adjacent the air injection well. However, in some instances it may be desirable to use separate wells for ignition and air injection. For example, it sometimes happens that the well utilized for ignition becomes damaged because of the extremely high temperatures developed. In this case, air may be injected through an offset injection well in order to establish an adequate combustion front and the combustion front moved through the formation by the continued injection of air through this offset well.

As the combustion front advances through the reservoir in the direction of the production well, gaseous combustion products including carbon monoxide, carbon dioxide, nitrogen (where air is used as the combustion-supporting medium), and water are driven through the reservoir ahead of the combustion front and the retort zone. These combustion products act as a displacing and heating medium with respect to the reservoir oil. As the reservoir oil is contacted by the combustion products, it is heated, thus effecting a viscosity reduction, and driven through the reservoir in the direction of the production well. In addition, the reservoir oil undergoes distillation and/or cracking in the vicinity of the retort zone and the distillation and cracking products are driven ahead of the combustion zone, also functioning as heating and displacing fluids.

During the initial phase of the combustion drive the production well is operated in a conventional manner to recover hydrocarbon fluids from the reservoir. At a suitable stage of the process, preferably after gas permeability between the injection and the production well has been established as described hereinafter, the production well is choked sutiiciently to effect an increase of atmospheres or more in the pressure within the interval of the well at the depth of the reservoir. The pressure within this interval commonly is termed the bottomhole pressure of the well and will be so designated in this description and in the appended claims. The production well may be throttled sufiiciently to completely shut it in such that no production is obtained during the time that the bottomhole pressure is being increased. Alternatively, the production well may be operated during this step at a reduced production rate so long as it is choked sufficiently to effect at least the specified bottomhole pressure increase.

As the bottomhole pressure of the production well increases, a corresponding pressure increase takes place within the reservoir adjacent to the production well. In response to this pressure increase, gas in the formation is forced into solution with the reservoir oil and, similarly, into solution with water which may be present in the reservoir. This dissolved gas has the effect of reducing the oil viscosity, swelling the oil, and also reducing the interfacial tension between the oil and water which may be present. During the time that the production well is choked, the pressure gradient in the reservoir from the injection well in the direction of the production well may be reduced. However, some pressure gradient will still exist so that there is some movement of the reservoir oil through the formation in the direction of the production oil.

After the production well has remained choked for the desired period of time, it is opened, with the attendant result that the bottomhole pressure of the well decreases. The decrease in bottomhole pressure is accompanied by a similar pressure decrease within the reservoir adjacent the production well. As this takes place the gas in liquid phase solution will tend to come out of solution and expand. Depending upon the rate of pressure decline, as described hereinafter, a portion of the dissolved gas will undergo bubble nucleation and expansion within the liquid phase. In this regard, a portion of the gas is surrounded by the liquid phase. As the pressure within the formation further decreases, the bubbles will tend to expand, while still remaining enclosed by the liquid phase, such that the oil or water within which such gas bubbles are contained increases in volume. As an exemplary illustration, consider a globule of oil containing dissolved gas and residing in an individual pore within the reservoir rock. As the pressure decreases, the globule of oil containing an expanding gas bubble is displaced out of the pore space. Thus, the oil may be displaced through the reservoir rock in the direction of the production well and the overall oil recovery is enhanced.

As noted previously, the tendency of the gas dissolved in the liquid phase to undergo bubble nucleation and expansion varies with the rate of pressure decrease. In this regard, the effect of bubble nucleation and expansion is enhanced by effecting a rapid pressure reduction. It is therefore preferred in carrying out the present invention to effect a rapid drawdown of the production well as it is opened to production.

An increase of 10 atmospheres during the choking step normally will be necessary in order to provide for a significant increase in oil recovery by the bubble nucleation and expansion mechanism described above. Thus, at a minimum, the choking step should be carried out so as to effect an increase in bottomhole pressure of at least 10 atmospheres. The rate at which the pressure will increase during the choking step will vary widely, depending upon the particular reservoir involved. In some cases the bottomhole pressure will be increased by 10 atmospheres, or considerably more than 10 atmospheres, within a period of several hours, particularly where the production well is completely shut in during the choking step. Even where a rapid pressure build-up is obtained it is preferred to continue the choking step for a period of at least one-quarter of a day in order to obtain an increase in pressure within the formation a significant distance from the production well. Even in reservoirs of relatively low permeability no advantage will be obtained by continuing the choking step for a period of more than thirty days. Thus, it will be preferred in carrying out the invention to choke the production well for a time period of at least one-quarter day but no longer than thirty days.

While the manipulation of the production well in accordance with the steps of the present invention may be undertaken at any time during the combustion process it is particularly beneficial after gas permeability has been established between the injection well and the production well. The gas-saturated zones will provide effective recovery avenues leading to the production well through which oil released by bubble nucleation and expansion may flow. Also, the combustion products present adjacent the production well when such gas permeability has been established are particularly effective in the bubble nucleation and expansion process. In this regard, the combustion products typically will include about 12% carbon dioxide, with a large portion of the remainder being nitrogen which was present in the originally injected air. The carbon dioxide component will be readily dissolved in the reservoir oil whereas the nitrogen component will tend to proceed through the reservoir under the influence of the existing pressure gradient, it being noted in this regard that the nitrogen component will be much less soluble in the reservoir oil than the carbon dioxide. The dissolved carbon dioxide produces expansion of the oil whereas the nitrogen will tend to act as a driving fluid, thus displacing the reservoir oil toward the production well. The nitrogen will function in this manner, both during the period in which the well is choked and also during the subsequent pressure drawdown step.

The stage of the combustion drive at which gas permeability between the injection and production wells is established can be ascertained by techniques well known to those skilled in the art. For example, the production well efiiuent may be monitored with regard to the pres ence of combustion products such as nitrogen and carbon dioxide and upon the detection of such products in measureable amounts it may be assumed that gas permeability between the injection and production wells has been established.

The introduction of combustion-supporting gas into the reservoir via the injection well should be continued during manipulation of the production well in accordance with the present invention. This will aid in the maintenance of a significant pressure gradient extending through the reservoir from the injection well to the production well with the attendant beneficial results noted hereinbefore.

The steps of choking the well and thereafter opening it to production may be repeated at appropriate intervals during the combustion drive until oil recovery becomes uneconomical. A typical history of a production well operated in accordance with the present invention is as follows. When the well is choked, the production rate will decrease immediately and, of course, will fall to zero if the well is shut-in. During this time the bottomhole pressure of the well will increase as described before. When the well is thereafter opened, after a bottomhole pressure increase of atmospheres or more, the pro duction rate will increase to a value greater than the rate existing immediately prior to the choking step. The production rate will reach a maximum, at which point it will then decrease with time.

As noted previously, the choking and opening steps may be repeated and this normally will be preferred in carrying out the present invention. The optimum repetition frequency of these steps will vary from reservoir to reservoir and from well to well, depending upon local conditions. It normally will be desirable to repeat the choking of the well only after the oil production rate during the opening step has declined from the maximum oil production rate by a value equivalent to at least 10% of the differential between the oil production rate observed immediately before choking of the well and the maximum oil production rate after opening of the well. Thus, for a well in which the production rate immediately before choking was ten barrels of oil per day and for which a maximum production rate of fifty barrels of oil per day was obtained upon opening of the well, the choking step should not thereafter be repeated until the production rate has declined by at least four barrels per day, i.e., to a level of forty-six barrels per day. While longer intervals between the repetitive steps of the invention may beand usually will beobserved, it is desirable to repeat the choking step before the production rate has declined by more than of the aforementioned production rate differential. Thus, in the example given above, the choking step should be repeated before the production rate has declined below fourteen barrels of oil per day.

It is preferred in carrying out the invention to repeat the choking step after the production rate has declined by a value within the range of 25% to 75% of the production rate differential between the oil production rate existing immediately before choking the well and the maximum oil production rate obtained after opening of the well. For most reservoir and well systems, operating within this range will result in the most economical rate of oil recovery with consideration given to production which is lost during the choking steps.

It is to be recognized that the foregoing criteria is to be applied with regard to the production rate differential existing before the well is first choked in and the maximum production rate obtained after opening of the well on each cycle of operation. Thus, when operating within the preferred range of 25% to 75% of the production rate differential in the example given above, the first opening step should be instituted when the oil production rate has declined to a value within the range of forty barrels per day (25 of the production rate differential of forty barrels per day) and twenty barrels per day (75% of the production rate differential). If upon the next subsequent opening step the oil production rate rises to a maximum of forty-six barrels per day, the subsequent choking step should be instituted when the oil production rate has declined to a rate within the range of thirty-seven barrels per day (25% of the production rate differential of thirty-six barrels per day) and nineteen barrels per day (75% of the production rate differential).

Having described specific embodiments of the instant invention it will be understood that further modifications thereof may be suggested to those skilled in the art, and it is intended to cover all such modifications as fall within the scope of the appended claims.

What is claimed is:

1. In the recovery of hydrocarbon fluids from a subterranean oil reservoir penetrated by an injection well and a production well, the method comprising:

(a) establishing a combustion front in said formation and advancing said combustion front through said formation in the direction of said production well by introducing a combustion-supporting gas through said injection well;

(b) recovering hydrocarbon fluids from said formation through said production well;

(0) choking said production well until the bottomhole pressure of said production well is increased by at least 10 atmospheres; and

(d) opening said production well and recovering hydrocarbon fluids therefrom as the bottomhole pressure of said well declines.

2. The method of claim 1, wherein said well is shut-in during step (c).

3. The method of claim 1, wherein said production well is choked in step (c) for a period of no more than thirty (30) days.

4. The method of claim 3, wherein said production well is choked in step (c) for a period of at least onequarter A) day.

5. The method of claim 1, wherein step (c) is initiated after gas permeability is established between said injection well and said production well.

6. The method of claim 1, further comprising repeating steps (c) and (d).

7. The method of claim 1, wherein hydrocarbon fluids are recovered in accordance wth step (d) until the oil production rate has declined from the maximum oil production rate obtained after opening the production well in step (d) by an amount equivalent to at least 10% of the production rate differential between the oil production rate existing before choking the production well in step (c) and said maximum oil production rate in step (d), and thereafter repeating steps (c) and (d).

8. The method of claim 7, wherein steps (c) and (d) are repeated before the oil production rate has declined from said maximum oil production rate in step (d) by more than 90% of said production rate differential.

9. The method of claim 1, wherein hydrocarbon fluids are recovered in accordance with step (d) until the oil production rate has declined from the maximum oil production rate obtained after opening the production well in step (d) by an amount within the range of 25% to 75% of the production rate differential between the oil production rate existing before choking the production well in step (c) and said maximum oil production rate in step (d), and thereafter repeating steps (c) and (d).

10. In the recovery of hydrocarbon fluids from a subterranean oil reservoir penetrated by an injection well and a production well, the method comprising:

(a) establishing a combustion front in said formation and advancing said combustion front through said formation in the direction of said production well by introducing a combustion-supporting gas through said injection well;

(b) recovering hydrocarbon fluids from said formation through said production well;

(0) after gas permeability is established between said injection well and said production well, choking said production well for a period within the range of one- 8 quarter A4) day to thirty (30) days and effecting an increase in the bottomhole pressure of said production well of at least 10 atmospheres; and

((1) opening said production Well and recovering hydrocarbon fluids therefrom as the bottomhole pressure of said well declines.

11. The method of claim 10, wherein hydrocarbon fluids are recovered in accordance with step (d) until the oil production rate has declined from the maximum oil production rate obtained after opening the production well in step (d) by an amount within the range of 25 to 75% of the production rate differential between the oil production rate existing before choking the production Well in step (c) and said maximum oil production rate in step (d), and thereafter repeating steps (c) and (d).

References Cited UNITED STATES PATENTS 2,390,770 11/1945 Barton et a1. 166-11 2,862,557 12/ 1958 Baron van Utenhove et al.

166-11 3,115,928 12/1963 Campion et al. 166-11 3,155,160 11/1964 Craig et al. 166-40 3,174,544 3/1965 Campion et al. 166-11 3,182,721 5/1965 Hardy 166-11 3,232,345 2/1966 Trantham et al. 166-11 X 3,280,910 10/1966 Crider 166-11 3,332,482 7/ 1967 Trantham 166-2 STEPHEN J. NOVOSAD, Primary Examiner.

US. Cl. X.R. 166-263, 272 

